Epithelial cells are arranged in sheets and provide the interface between the external world and the internal environment (i.e., ECF) of the body. Depending on their location, epithelial cells serve many important functions, such as establishing a barrier to microorganisms (lungs, gastrointestinal tract, and skin), prevention of the loss of water from the body (skin), and maintenance of a constant internal environment (lungs, gastrointestinal tract, and kidneys). This latter function is a result of the ability of epithelial cells to carry out regulated vectorial transport (i.e., transport from one side of the epithelial cell sheet to the opposite side). In this section, the principles of epithelial transport are reviewed. The transport functions of specific epithelial cells are discussed in the appropriate chapters throughout this book.
Fig. 2.8 shows a schematic representation of an epithelial cell. The free surface of the epithelial layer is referred to as the apical membrane. It is in contact with the external environment (e.g., air within the alveoli and larger airways of the lungs and the contents of the gastrointestinal tract) or with extracellular fluids (e.g., glomerular filtrate in the nephrons of the kidneys and the secretions of the ducts of the pancreas or sweat glands). The basal side of the epithelium rests on a basal lamina, which is secreted by the epithelial cells, and this in turn is attached to the underlying connective tissue.
Epithelial cells are connected to one another and to the underlying connective tissue by a number of specialized junctions (see
Fig. 2.8 ). The adhering junction, desmosomes, and hemidesmosomes provide mechanical adhesion by linking together the cytoskeleton of adjacent cells (adhering junction and desmosome) or to the underlying connective tissue (hemidesmosome). The gap junction and tight junction play important physiological roles.
Gap junctions provide low-resistance connections between cells.
The functional unit of the gap junction is the connexon. The connexon is composed of six integral gGap junctions are not limited to epithelial cells. A number of other cells also have gap junctions (e.g., cardiac myocytes and smooth muscle cells).
•Fig. 2.8 Schematic of an Epithelial Cell, Illustrating the Various Adhering Junctions. Thetightjunctionseparatestheapicalmembranefrombasolateralmembrane(seetextfordetails).
membrane protein subunits called connexins. A connexon in one cell is aligned with the connexon in the adjacent cell, forming a channel. The channel may be gated, and when it is open, it allows the movement of ions and small molecules between cells. Because of their low electrical resistance, they effectively couple electrically one cell to the adjacent cell.
The tight junction serves two main functions. It divides the cell into two membrane domains (apical and basolateral) and, in so doing, restricts the movement of membrane lipids and proteins between these two domains. This so-called fence function allows epithelial cells to carry out vectorial transport from one surface of the cell to the opposite surface by segregating membrane transporters to one or other of the membrane domains. They also serve as a pathway for the movement of water, ions, and small molecules across the epithelium. This pathway between the cells is referred to as the paracellular pathway, as opposed to the transcellular pathway through the cells.
The apical surface of epithelial cells may have specific structural features. One such feature is microvilli (Fig.